The present invention relates to a cellular mobile radio communication system including base stations that send pilot signals all the time and a mobile station or stations each adapted to receive the pilot signals to choose the base station with the strongest pilot signal power among all received pilot signals. More specifically, the present invention relates to a pilot signal reception level averaging system for precisely determining a propagation loss between the base stations and the mobile station.
A cellular mobile radio communication system is known as an example of the cellular mobile radio communication system. Such a system includes a number of radio base stations scattered therein and they use the same frequency range at the same time. On the other hand, the mobile station normally establishes connection between one of the base station with the minimum propagation loss. The mobile station establishes communication with the base station with the minimum transmission power by means of transmission power control. The transmission power control minimizes interference to any other radio channels and thus increased number of mobile stations can access the particular base station simultaneously. The base stations transmit pilot signals to the mobile station all the time. The mobile station estimates the propagation loss from the received pilot signal. The pilot signal serves as an index for the transmission power control or choice of the base station that the mobile station accesses.
The reception level at which the mobile station receives the pilot signals fluctuates due to a propagation loss and a fading component overlapped with the propagation loss. The reception level for the pilot signal with the overlapped fading fluctuation component increases an error in transmission power control when the mobile station performs the transmission power control or chooses the base station. Such an error may cause improper choice of the base station of the minimum propagation loss for the mobile station in question. Therefore, the propagation loss component is acquired by means of averaging the reception levels at which the pilot signals are received.
Referring to FIG. 1, an averaging technique used conventionally is described. A pilot signal of a reception level P passes through n number of delay circuits 100-1 through 100-n. A summing unit 200 calculates an average of the reception levels at which the n number of past pilot signals are received. The summing unit 200 thus obtains an averaged pilot signal reception level P.sub.0.
FIG. 2 is a graphical representation of the above-mentioned averaging operation. At the beginning of observation, the reception levels at which the pilot signals are received is suffered from level fluctuation with the fading fluctuation component overlapped with the propagation loss component. Then, the fading fluctuation component is limited by means of the averaging. The resultant reception level is well in proportion with the propagation loss component.
However, referring to FIGS. 3 and 4, a fluctuation profile for the reception level at which the pilot signal is received changes depending on various factors including a moving speed and a transmission environment of the mobile station. Accordingly, there is a case that the correct propagation loss component can not be acquired, even if the pilot signals received is averaged constantly. For example, the reception level for the pilot signals that is averaged with a conventional averaging technique is not significantly different from the reception level before averaging with a relatively sluggish fading fluctuation profile. On the contrary, rapid fluctuation of the propagation loss results in excessive averaging, which causes averaging of the propagation loss fluctuation as well.
As described above, the conventional technique cannot provide precise and correct acquirement of the propagation loss component out of the pilot signal reception level. This may cause incorrect choice of the base station by the mobile station and undesirable increase of the transmission power control error. The problem is particularly significant in the code division multiple access cellular mobile radio communication system because incorrect choice of the base station with the minimum propagation loss results in the interference to other radio channels, decreasing the total number of mobile stations that the communication system can handle.